1. Technical Field
The disclosure relates to an exhaust gas control apparatus for an internal combustion engine.
2. Description of Related Art
An internal combustion engine that is provided with a catalyst which purifies exhaust gas of nitrogen oxide (NOx) is known (for example, refer to Japanese Patent Application Publication No. 2014-88800 (JP 2014-88800 A)). An adding mechanism that adds urea water to the exhaust gas is disposed in an exhaust passage of the internal combustion engine, and ammonia generated from the urea water is adsorbed onto the NOx-removing catalyst. Then, the NOx is removed by reduction by the ammonia adsorbed on the catalyst.
When an insufficient amount of the ammonia is adsorbed on the catalyst, the NOx removal cannot be appropriately performed. An excessive ammonia adsorption amount, meanwhile, is likely to result in a phenomenon known as “ammonia slip.” Accordingly, it is desirable to set a target adsorption amount for the ammonia adsorbed onto the catalyst and to execute an ammonia adsorption amount control for controlling the amount of the urea water addition based on the target adsorption amount.
Regarding the execution of the ammonia adsorption amount control, it is important to suppress an error between the target adsorption amount and an actual ammonia adsorption amount. However, ammonia desorption and adsorption are repeated in the catalyst as a result of the NOx removal, and this causes the actual ammonia adsorption amount to deviate from the target adsorption amount. Such deviation causes the error (i.e., the difference between the target and actual ammonia adsorption amounts) to accumulate.
In the device disclosed in JP 2014-88800 A or the like, an initialization processing is performed in this regard when the deviation between an estimated value of the ammonia adsorption amount and the actual ammonia adsorption amount is equal to or higher than a predetermined value. This initialization processing causes the ammonia to be fully desorbed from the catalyst by executing a process of raising a temperature of the exhaust gas to cause a temperature of the catalyst to rise. When the initialization processing is performed, the accumulated error is eliminated, and thus the deviation of the actual ammonia adsorption amount with respect to the target adsorption amount can be suppressed during the subsequent ammonia adsorption amount control.